A stem cell is commonly defined as a cell that (i) is capable of renewing itself; and (ii) can give rise to more than one type of cell through asymmetric cell division (Watt et al., Science, 284:1427-1430, 2000). Stem cells typically give rise to a type of multipotent cell called a progenitor cell; progenitor cells, in turn, proliferate and differentiate into lineage-committed cells that populate the body.
Pluripotent stem cells are thought to have the potential to differentiate into almost any cell type, while multipotent stem cells are believed to have the potential to differentiate into many cell types (Robertson, Meth. Cell Biol. 75:173, 1997; Pedersen, Reprod. Fertil. Dev. 6:543-552, 1994).
Stem cells exist in many tissues of embryos and adult mammals. Many different types of mammalian stem cells have been characterized and certain stem cells have not only been isolated and characterized, but have also been cultured under conditions to allow differentiation to a limited extent. Both adult and embryonic stem cells are able to differentiate into a variety of cell types and, accordingly, may be a source of replacement cells and tissues that are damaged in the course of disease or infection, or because of congenital abnormalities (Lovell-Badge, Nature 414:88-91, 2001; Donovan et al., Nature 414:92-97, 2001). Various types of putative stem cells exist which, when differentiated into mature cells, carry out the unique functions of particular tissues, such as heart, liver, or neuronal tissue. These cells are important for the treatment of a wide variety of disorders, including malignancies, inborn effors of metabolism, hemoglobinopathies, immunodeficiencies and the replacement of damaged and diseased tissues. Recent success at transplanting such stem cells have provided new clinical tools to reconstitute and/or supplement bone marrow after myeloablation due to disease, exposure to toxic chemical and/or radiation. Further evidence exists that demonstrates that stem cells can be employed to repopulate many, if not all, tissues and restore physiologic and anatomic functionality. The application of stem cells in tissue engineering, gene therapy delivery and cell therapeutics is also advancing rapidly.
Prior to the present invention, a basic problem existed, i.e., that obtaining sufficient quantities and populations of human stem cells which are capable of differentiating into most cell types was nearly impossible. Stem cells are in critically short supply.
Obtaining sufficient numbers of human stem cells has been problematic for several reasons. First, isolation of normally occurring populations of stem cells in adult tissues has been technically difficult and costly due, in part, to very limited quantity found in blood or tissue. The isolation of the cells is generally laborious, involving the harvesting of cells or tissues from a patient or donor, culturing and/or propagating the cells in vitro. Certain cell types, such as nerve cells and cardiac cells, differentiate during development, and adult organisms are not known to generally replace these cells. Even in cell types that are replaced in adult organisms (e.g., epithelial cells and hematopoietic cells), it has been a significant challenge to readily and inexpensively obtain stem cells in significant quantities. For example, mammalian hematopoietic cells (e.g., lymphoid, myeloid, and erythroid cells) are all believed to be generated by a single cell type called the hematopoietic “stem cell” (Civin et al., J. Immunol. 133:157-165, 1984). However, these hematopoietic stem cells are very rare in adults, accounting for approximately 0.01% of bone marrow cells. Isolation of these cells based on surface proteins such as CD34 results in very low yields. Schemes to fractionate human hematopoietic cells into lineage committed and non-committed progenitors are technically complicated and often do not permit the recovery of a sufficient number of cells to address multilineage differentiation (Berenson et al., 1991; Terstappen et al., 1991; Brandt et al., J. Clin. Invest. 82:1017-1027, 1988; Landsdorp et al., J. Exp. Med. 175:1501-1509, 1992; Baum et al., Proc. Natl. Acad. Sci. USA 89:2804-2808, 1992)
A second reason that obtaining sufficient number of human stem cells has been problematic is that procurement of these cells from embryos or fetal tissue has raised religious, ethical, and legal concerns. Alternative sources that do not require the use of cells procured from embryonic or fetal tissue are therefore highly desirable for clinical use of stem cells. Prior to the present invention, there have been few viable alternative sources of stem cells, particularly human stem cells.
It would therefore be of particularly great value in treating a wide variety of diseases to have an easily accessible quantity of embryonic like stem cells that are found in the adult body that can reliably differentiate into a desired phenotype.
In addition, it would also be advantageous to have stem cells that do not require feeder cells. Many adult stem cell propagation protocols require such cells, which creates risks including infection, cell fusion, and/or contamination. As such, adult stem cells are have often been very difficult to expand in culture (Reya et al., Nature 414:105-111, 2001; Tang et al. Science 291:868-871, 2001).
Thus, there remains a need in the art to develop methods for identifying, propagating, and altering the state (e.g., by differentiation or dedifferentiation) of stem cells and to provide a source of cells that are transplantable to in vivo tissues in order to replace damaged or diseased tissue.